A fuel cell stack is a power generation system which oxidizes a fuel by means of an electrochemical process to emit energy as a result of an oxidation reaction and which converts the resulting energy directly into electric energy. The fuel cell stack includes a membrane-electrode assembly including a polyelectrolyte membrane through which hydrogen ions are selectively transported and paired electrodes made up of a porous material and sandwichingly holding opposite side surfaces of the polyelectrolyte membrane. Each of the paired electrodes is mainly composed of carbon power carrying a platinum-containing metal catalyst and includes a catalyst layer that is in contact with the polyelectrolyte membrane and a gas diffusion layer formed on a surface of the catalyst layer and which is both air permeable and electronically conductive.
A fuel cell vehicle with the fuel cell system mounted therein as a power source controls operation of the fuel cell system such that in a high power region with a high power generation efficiency, the fuel cell stack generates power to allow both the fuel cell stack and a secondary battery or only the fuel cell stack to supply power to a traction motor and such that in a low power region with a low power generation efficiency, the power generation by the fuel cell stack is halted to allow only the secondary battery to supply power to the traction motor. Thus halting the operation of the fuel cell stack in the low load region in which the fuel cell system offers the reduced power generation efficiency is called an intermittent operation. In the low load region in which the fuel cell system offers the reduced power generation efficiency, the intermittent operation is performed to enable the fuel cell stack to operate with the energy conversion efficiency kept high. The efficiency of the whole fuel cell system can thus be increased.
Japanese Patent Laid-Open No. 2004-172028 refers to a fuel cell system that performs the intermittent operation when a load requirement for the fuel cell stack is equal to or lower than a predetermined value. Japanese Patent Laid-Open No. 2004-172028 also refers to a method of preventing a possible delay in a response to a generation request by, when the cell voltage of the fuel cell stack changed into a generation halt state as a result of the intermittent operation is lower than a predetermined value, driving an air compressor to supply an oxygen gas to the fuel cell stack, thus avoiding the possible shortage of oxygen in a cathode of the fuel cell stack to recover the cell voltage.    [Patent Document 1] Japanese Patent Laid-Open No. 2004-172028